Solar Wind-magnetosphere Energy Coupling Research Articles

Establishment and Application of an Interplanetary Disturbance Index Based on the Solar Wind–Magnetosphere Energy Coupling Function and the Spectral Whitening Method

We develop a preliminary interplanetary disturbance index (J sW ) by applying the spectral whitening method to an energy coupling function with solar wind measurements during the years 1998–2014 as the input, which can be used as an indicator of perturbations in the near-Earth solar wind. The correlation and temporal variation between J sW and the geomagnetic disturbance index (J pG ) constructed from the same method have been analyzed in detail for 167 geomagnetic storms with the minimum D st index less than or equal to −50 nT. The time delay between J sW and J pG is clearly observable and varies for different events, according to which J sW is shifted backward with respect to J pG . We obtain a fairly good negative correlation between the shifted J sW and the J pG indices for the majority of events, and the significance level for 88% of the events (i.e., 147 events) does not exceed 0.05. A statistical analysis of the shifted J sW and the J pG indices for 147 selected events reveals that larger values of J sW and smaller magnitudes of J pG are commonly accompanied by enhanced southward magnetic fields, which implies that more solar wind energy is entering the magnetosphere and thus causing strong geomagnetic storms. Furthermore, a linear fit of the two indices suggests that the evolution of J pG can be predicted about 2 hr in advance based on J sW , indicating that J sW can provide early warnings of possible disturbances in the geomagnetic fields, which is crucial for space weather monitoring and operational forecasting.

Weakest Solar Cycle of the Space Age: A Study on Solar Wind–Magnetosphere Energy Coupling and Geomagnetic Activity

Solar Cycle 24, from December 2008 to December 2019, is recorded to be the weakest in magnitude in the space age (after 1957). A comparative study of this cycle with Solar Cycles 20 through 23 is presented. It is found that Solar Cycle 24 is not only the weakest in solar activity, but also in average solar wind parameters and solar wind–magnetosphere energy coupling. This resulted in lower geomagnetic activity, lower numbers of high-intensity long-duration continuous auroral electrojet ( $AE$ ) activity (HILDCAA) events and geomagnetic storms. The Solar Cycle 24 exhibited a $\approx 54$ – $61\%$ reduction in HILDCAA occurrence rate (per year), $\approx 15$ – $34\%$ reduction in moderate storms ( $-50~\text{nT} \geq Dst > -100~\text{nT}$ ), $\approx 49$ – $75\%$ reduction in intense storms ( $-100~\text{nT} \geq Dst > -250~\text{nT}$ ) compared to previous cycles, and no superstorms ( $Dst \leq -250~\text{nT}$ ). Implications of the solar and geomagnetic weakening to space weather science and operations are discussed.

Diminishing activity of recent solar cycles (22–24) and their impact on geospace

This study examines the variation of different energies linked with the Sun and the Earth’s magnetosphere-ionosphere systems for solar cycles (SCs) 22–24 for which the gradual decrease in the solar activity is noticed. Firstly, we investigated the variation of solar magnetic energy density (SMED) for SCs 21–24 and its relation to the solar activity. We observed distinct double peak structures in SMED for the past four SCs, 21–24. This feature is consistent with noticeable asymmetry in their two peaks. For SCs 22–24 a significant decrease is observed in the integrated SMED of each SC. This reduction is 37% from SCs 22 to 23 and 51% from SCs 23 to 24, which indicates substantial weakening of Sun’s magnetic field for SC 24. Also, the magnetic, kinetic, and thermal energy densities at the Earth’s bow-shock nose are found to be considerably low for the SC 24. We examined the solar wind Alfven speed, magnetosonic Mach number, solar wind-magnetosphere energy coupling parameter (ε), and the Chapman-Ferraro magnetopause distance (LCF) for the SCs 22–24. The estimated maximum stand-off magnetopause distance is larger for SC 24 (LCF ≤ 10.6 RE) as compared to SC 23 (LCF ≤ 10.2 RE) and SC 22 (LCF ≤ 9.8 RE). The solar wind Alfven speeds during SCs 22 and 23 are in the same range and do not exceed ≈73 km/s whereas, it is below 57 km/s for SC 24. A lower bound of solar wind magnetosonic Mach number for SC 24 is larger (M ≥ 6.9) as compared to SC 22 (M ≥ 5.9) and SC 23 (M ≥ 6). We noticed weakening in the energy coupling parameter for SC 24, which resulted in substantial (15%–38%) decrease in average strength of high latitude ionospheric (AE), low latitude magnetospheric (Dst) and equatorial ionospheric (EEJ) current systems in comparison with SC 23. Subsequently, a reduction of ≈30% is manifested in the high latitude Joule heating for SC 24. Overall this study indicates the significant step down in various energies at Sun, Earth’s bow-shock, and near Earth environment for current SC 24, which will have important implication on our Earth’s atmosphere-ionosphere-magnetosphere system.

Multi-satellite observations of energy transport during an intense geomagnetic storm

Energy transport during a geomagnetic substorm is a very important process for solar wind-magnetosphere energy coupling and the energy cycle in the magnetotail. In this paper, we use magnetotail data from the five THEMIS probes and two Cluster satellites on the dayside to investigate the energy transport of one intense storm during the period from 08 March to 11 March 2008 at large spatial-temporal scales. Simultaneous observations of the five THEMIS probes indicate that there is a stronger and earlier duskward energy flux density in the near-Earth magnetotail than that in the mid-tail in the initial phase. Low energy particles inject earthward from the dusk flank. Stronger and more variable earthward energy flux density is observed in the mid-tail compared to that near Earth in the main phase; mainly caused by high-speed flow. Tailward energy flux was observed in the near-Earth and mid-tail regions during the recovery phase. Dayside data observed by two Cluster satellites show that the duskward energy flux may be related to stable solar wind input. Tailward energy flux on the dayside should experience some energy conversion process in the magnetotail before it can provide the earthward energy flux in the magnetotail for this intense storm. The strongest energy transport observed by the nightside probes occurs in the main phase. However, the strongest energy measured by the dayside satellites is in the recovery phase without intense activities, two hours later. Different features of the energy transport in the three phases of the storm may be closely related to the different physical processes such as the energy entry, westward drift, particle injection or other potential mechanisms.

The importance of ground magnetic data in specifying the state of magnetosphere–ionosphere coupling: a personal view

In the history of geomagnetism, geoelectricity and space science including solar terrestrial physics, ground magnetic records have been demonstrated to be a powerful tool for monitoring the levels of overall geomagnetic activity. For example, the Kp and ap indices having perhaps the long-history geomagnetic indices have and are being used as space weather parameters, where “p” stands for “planetary” implying that these indices express average geomagnetic disturbances on the entire Earth in a planetary scale. To quantify the intensity level of geomagnetic storms, however, it is common to rely on the Dst index, which is supposed to show the magnitude of the storm-time ring current. Efforts were also made to inter-calibrate various activity indices. Different indices were proposed to express different aspects of a phenomenon in the near-Earth space. In the early 1980s, several research groups in Japan, Russia, Europe and the US developed the so-called magnetogram-inversion techniques, which were proposed all independently. Subsequent improvements of the magnetogram-inversion algorithms allowed their technology to be applied to a number of different datasets for magnetospheric convection and substorms. In the present review, we demonstrate how important it was to make full use of ground magnetic data covering a large extent in both latitudinal and longitudinal directions. It is now possible to map a number of electrodynamic parameters in the polar ionosphere on an instantaneous basis. By applying these new inverse methods to a number of ground-based geomagnetic observations, it was found that two basic elements in spatial patterns can be viewed as two physical processes for solar wind-magnetosphere energy coupling.

Influence of Solar Wind on the TEC Variations at Mid and Sub-Auroral Latitudes During Sunspot Maximum

One cannot find many studies on the relationship between the amount of energy transferred in to the earth’s magnetosphere and ionospheric TEC variations observed at mid or sub-auroral latitudes. In this work we have attempted to relate day-to-day variations of four well known solar wind-magnetosphere energy coupling parameters and daily mean TEC variations at a mid latitude (Sagamore Hill) and a sub-auroral (Goose bay) station during the year 1981, at sunspot maximum. We could obtain statistically significant correlation between the variations of the above solar wind parameters and TEC at Goose bay and Sagamore Hill during the months February (winter) and March (equinox) for the year of study. However we cannot find a similar result for August (summer) 1981. This study demonstrates the usefullness of solar wind-magnetospheric coupling parameters in modelling ionospheric TEC variations during high solar activity periods.

Numerical study on ULF waves in a dipole field excited by sudden impulse

A three-dimensional numerical model is employed to investigate ULF waves excited by the sudden impulse (SI) of the solar wind dynamic pressure interacting with a dipole magnetosphere. We focus on the solar wind-magnetosphere energy coupling through ULF waves, and the influences of the SI spectrum on the cavity mode structure and the energy deposition due to field line resonances (FLRs) in the magnetosphere. The numerical results show that for a given SI lasting for 1 min with amplitude of 50 mV/m impinging on the subsolar magnetopause, the total ULF energy transported from the solar wind to the magnetosphere is about the magnitude of 1014 J. The efficiency of the solar wind energy input is around 1%, which depends little on the location of the magnetopause in the model. It is also found that the energy of the cavity mode is confined in the region near the magnetopause, whereas, the energy of the toroidal mode may be distributed among a few specific L-shells. With a given size of the model magnetosphere and plasma density distribution, it is shown that the fundamental eigenfrequency of the cavity mode and the central locations of the FLRs do not vary noticeably with the power spectrum of the SI. It is worth noting that the spectrum of the SI affects the excitation of higher harmonics of the global cavity mode. The broader the bandwidth of the SI is, the higher harmonics of cavity mode could be excited. Meanwhile, the corresponding FLRs regions are broadened at the same time, which implies that the global cavity modes and toroidal modes can resonate on more magnetic L-shells when more harmonics of the global cavity modes appear.

Apposite of atmospheric electric parameters with the energy coupling function ( ε) during geomagnetic storms at high latitude

In this paper we are concerned with the variation of the atmospheric electric field and the air–earth current due to the excessive power generated by the solar wind-magnetosphere dynamo during geomagnetic storms, recorded at Maitri in Antarctica during 2004. A major part of the power generated by the solar wind-magnetosphere dynamo is used in the formation of the ring current and the rest is utilized for Joule heating and auroral particle precipitation. The method adopted by Frank-Kamenetsky et al. [Frank-Kamenetsky A.V., Troshichev O.A., Burns G.B., Papitashvili V.O., 2001. Variations of atmospheric electric field in the near-pole region related to the interplanetary magnetic field. J. Geophys. Res. 106, A1, 179–190.] was utilized to delineate the variations due to the signatures of tropical thunderstorm activity from the geoelectric data; while statistical methods used in our earlier studies were used to delimit variation due to the constant buffeting of the solar wind. We find that the solar wind-magnetosphere energy coupling function ( ε) to be well correlated with the atmospheric electric parameters during the onset of geomagnetic disturbances. However the correlation breaks down during minor storms and sub-storm events.

준 주기적인 서브스톰과 고속 태양풍 사이의 에너지 결합에 대한 정량적 평가

태양활동이 감소하는 시기에는 코로나 구멍(Coronal hole)과 관련된 고속 태양풍(High-speed stream)이 빈번하게 발생하는데, 고속 태양풍 기간 동안에는 서브스톰(Substorm)이 대략 <TEX>$2{\sim}4$</TEX>시간 간격으로 반복적으로 발생한다. 이 연구에서는 태양활동이 감소하는 시기 중 2003년 후반기 즉, 7월부터 12월 사이의 고속 태양풍 기간 동안에 반복적으로 발생한 222건의 서브스톰을 선정하여 연속한 두 서브스톰 사이의 태양풍 에너지 유입 정도를 정량적으로 산출하였다. 이를 위해 Akasofe <TEX>${\varepsilon}$</TEX>-parameter를 인접한 두 서브스톰 발생시간에 대해 시간 적분하여 한 서브스톰이 발생 후 다음 서브스톰이 발생하기 전까지의 에너지 유입량을 산출하였다. 이와 같은 방법을 고속 태양풍 기간 동안에 반복적으로 발생한 222건의 서브스톰에 대해 적용하여 통계를 산출하였다. 그 결과로서 두 서브스톰 사이의 평균 태양풍 에너지 유입량은 <TEX>$1.28{\times}10^{14}J$</TEX>이고, 전체 서브스톰 이벤트 중 약 85%가 <TEX>$2{\times}10^{13}{\sim}2.3{\times}10^{14}J$</TEX> 범위에 있음을 알 수 있었다. 따라서 두 서브스톰 사이의 태양풍 에너지 유입량은 경우에 따라 상당히 다른 값을 가질 수 있는 것으로 보이며, 태양풍 에너지 유입량만으로 서브스톰이 발생하는 시점을 예측하기에는 어려움이 있다고 생각된다. 더불어 이 논문에서는 연속적인 서브스톰의 발생에 영향을 미치는 여러 요소에 대한 논의를 제공한다. It has been known that high-speed solar wind streams associated with coronal holes lead to quasi-periodic substorms that occur approximately every <TEX>$2{\sim}4$</TEX> hours. In this paper we examined 222 repetitive substorms that occurred during high-speed stream periods in July through December in 2003 to quantitatively determine a range of energy input from the solar wind into the magnetosphere between two consecutive substorms. For this study, we have used the Akasofu <TEX>${\varepsilon}$</TEX>-parameter to time-integrate it for the interval between two consecutive substorms, and have applied this method to the 222 substorms. We find that the average amount of solar wind input energy between two adjacent substorms is <TEX>$1.28{\times}10^{14}J$</TEX> and about 85% out of the 222 substorms occur after an energy input of <TEX>$2{\times}10^{13}{\sim}2.3{\times}10^{14}J$</TEX>. Based on these results, we suggest that it is not practical to predict when a sub storm will occur after a previous one occurs purely based on the solar wind-magnetosphere energy coupling. We provide discussion on several possible factors that may affect determining substorm onset times during high-speed streams.

Solar wind–magnetosphere energy coupling under extreme interplanetary conditions: MHD simulations

In this paper, we present results derived from global magnetohydrodynamic simulations of the response of the magnetosphere to magnetic clouds. The simulations were driven with solar wind data from 10 January 1997 and 4 May 1998. Both periods were major magnetic storms. We find that during moderate periods of magnetic merging between the interplanetary magnetic field (IMF) and the magnetospheric magnetic field, the simulated magnetosphere responds with a loading–unloading behavior characteristic of substorms. However, during periods of strong, sustained, southward IMF, the simulation evolves to a directly driven state, without the loading–unloading component. This behavior is the same as that of the real magnetosphere (Bargatze, L.F., Baker, D.N., McPherron, R.L., Hones, E.W., Jr., 1985. Magnetospheric response for many levels of geomagnetic activity. J. Geophys. Res., 90, 6387–6394), thus providing compelling evidence that the simulation is accurately representing the fundamental features of solar wind–magnetosphere coupling.

Influence of interplanetary interaction regions on geomagnetic disturbances and tropospheric circulation

Proposed solar wind-magnetosphere energy coupling functions are studied. An empirical formula proposed by Svalgaard (1977) is found to predict the geomagnetic activity quite well. The influence of solar wind interaction regions on the tropospheric circulation, through a suggested cosmic ray mechanism, was investigated. The cosmic ray intensity at Earth clearly showed a decrease at the time of passage of an interaction region. It is suggested that the well-known dip in the Vorticity Area Index may be caused by an interaction-modulated decrease in cosmic ray intensity.

Discussion of “A high time resolution study of the solar wind-magnetosphere energy coupling function” by Akasofu, Carbary, Meng, Sullivan and Lepping

Discussion sur le temps de delais entre la fonction de couplage d'energie et les perturbations ionosphere/magnetosphere indiquees par l'indice AE

Evolution of ideas in solar-terrestrial physics

Summary. This essay is an attempt to follow, from the beginning of this century, the evolution of various concepts developed in the field of the solar activity-magnetospheric disturbance relationship, but is not merely a chronological description. In most scientific fields, as observed by Thomas S. Kuhn, there is a strong tendency for scientists to have a high degree of agreement on theoretical assumptions establishing a coherent tradition of research, defined as a ‘paradigm'. Some paradigms are often initiated by a simple, qualitative intuition. However, once established, they are articulated and elaborated to the point that observational or theoretical anomalies are treated not as refuting the underlying theory, but as solvable puzzles by ad hoc modification to the original theory. Solar-terrestrial physics is no exception in this regard. We examine the evolution of various disciplines in solar-terrestrial physics from this particular point of view in the following disciplines: (1) the solar wind generation, (2) the source regions of the high speed streams, (3) solar flares, (4) the magnetosphere formation, (5) the morphology of magnetospheric disturbances, (6) the solar wind-magnetosphere energy coupling, and (7)auroral particle acceleration. It is also pointed out that the tendency toward increasing specialization in various disciplines tends to create a gap among interrelated disciplines. This is particularly serious in solar-terrestrial physics, since a study of the north-south component of the interplanetary magnetic field has fallen into the gap between solar physics and magnetospheric physics.

Ring current energy injection rate and solar wind-magnetosphere energy coupling

Assuming that the formation of the ring current belt is a direct consequence of an enhanced convection of plasma sheet protons, the expression for the energy injection rate U R is formulated as a function of the cross-tail potential drop φ CT for a simple electric field-magnetic field model. It is shown that an approximate expression for U R thus formulated consists of two parts: (i) the first part U R1 , which is linearly proportional to φ CT , is supplied by the corotation electric field and (ii) the second part U R>2 , which is proportional to φ 2 CT , is supplied by the solar wind energy input to the magnetosphere. The second part U R2 dominates the ring current energy input when the cross-tail potential drop φ CT is greater than ∼ 95 keV, namely during disturbed periods. An important finding is that the second part U R2 of the ring current energy input is shown to be proportional to the solar wind-magnetosphere energy coupling function ϵ, recovering the observationally established relationship. Therefore, the present study verifies that an enhanced convection is the cause of the ring current formation.

A high time resolution study of the solar wind-magnetosphere energy coupling function

A high time resolution study of the relationship between the solar wind-magnetosphere energy coupling function ϵ and the total energy dissipation rate U T of the magnetosphere is made using 5-min average values of solar wind data and of the geomagnetic indices AE and Dst. All the results are essentially the same as those obtained by the earlier studies which were based on the hourly average data set. Therefore, we confirm that the magnetosphere is primarily a driven system.

Dynamo process governing solar wind-magnetosphere energy coupling

Based on the method of dimensional analysis, the energy transfer rate from the solar wind into the magnetosphere can be characterized by a magnetic coupling parameter α on open field lines and by a viscous coupling parameter β on closed field lines. By assuming that the energy transfer rate can be monitored by the total energy dissipation rate of the magnetosphere, the histogram of α is constructed and is found to peak around −0.1 < α < 0.1. This result implies that the energy transfer is governed primarily by the MHD dynamo process on open field lines and indicates that the ϵ function obtained by Perreault and Akasofu is verified as the first approximation of the solar wind-magnetosphere energy coupling function.

Simulation of three-dimensional solar wind disturbances and resulting geomagnetic storms

A kinematic method of representing the three-dimensional solar wind flow is devised by taking into account qualitatively the stream-stream interaction which leads to the formation of a shock pair. Solar wind particles move radially away from the Sun, satisfying the frozen-magnetic field condition. The uniqueness of the present approach is that one can incorporate both theoretical and observational results by adjusting the parameters involved and that a self-consistent data set can be simulated. One can then infer the three-dimensional structure of the solar wind which is vital in understanding the interaction between the solar wind and the magnetosphere, and it is for this reason that the present kinematic method is devised. In the first part of this paper, the present kinematic method is described in detail by demonstrating that the following solar wind features can be simulated: (i) Variations of the solar wind quantities (such as the solar wind speed, the density and the IMF vector), associated with the solar rotation, at the Earth; (ii) the solar wind flow pattern in the meridian planes; (iii) the three-dimensional structure of the corotating interaction region (CIR); and (iv) the three-dimensional structure of the warped solar current sheet. In Section 2, the three-dimensional structure of solar wind disturbances are studied by introducing a flare-generated high speed stream into the two-stream model of the solar wind developed in Section 1. The treatment of the stream-stream interaction is generalized to deal with a flare-generated high speed stream, yielding a shock pair. The shock pair causes three-dimensional distortion of the solar current sheet as it propagates outward from the Sun. It is shown that a set of characteristic time variations of the solar wind speed, density, the interplanetary magnetic field magnitude B and angles Θ (theta) and gf (phi) result at the time of the passage at the location of the Earth for a given set of flare conditions. These quantities allow us to compute the solar wind-magnetosphere energy coupling function ɛ. Time variations of the two geomagnetic indices AE and Dst are then estimated from ɛ. The simulated geomagnetic storms are compared with observed ones. In the third part, it is shown that recurrent geomagnetic storms can reasonably be reproduced, if fluctuating components of the interplanetary magnetic field (IMF) are superposed on the kinematic model of the solar wind developed in the first part. As an example, we simulate the fluctuating components by linearly polarized Alfven waves and by random variations of the IMF angle Θ (theta). Characteristics of the simulated and observed geomagnetic storms are discussed in terms of the simulated and observed AE and Dst indices. If the fluctuating components of the IMF can generally be identified as hydromagnetic waves, they may be an important cause for individual magnetospheric substorms, while the IMF magnitude B and the solar wind speed V modulate partially the intensity of magnetospheric substorms and storms.

Prediction of development of geomagnetic storms using the solar wind-magnetosphere energy coupling function ϵ

It is shown that by monitoring time variations of the solar wind-magnetosphere energy coupling function ϵ( t) upstream of the solar wind, one should be able to predict fairly accurately the growth and decay of individual magnetospheric substorms and storms.

Magnetospheric substorms: A newly emerging model

A surge of progress in magnetospheric substorm studies is expected by the following three recent developments, (i) the finding of the solar wind-magnetosphere energy coupling function ϵ, (ii) the determination of the Pedersen current distribution over the entire polar region, and (iii) a new understanding of the auroral potential structure. In this paper, the significance of the three developments and the newly emerging model of magnetospheric substorms is described.

Passage of the solar current disk and major geomagnetic storms

It is shown that major geomagnetic storms (¦Dst¦ > 100γ) tend to develop at about the time of the passage of the solar current sheet or disk at the location of the Earth, provided this passage is associated with (1) a large impulsive increase of the IMF magnitude B, (2) a negative value of the IMF angle Θ (Theta), and (3) an increasing solar wind speed. The passage occurs in association with the 27-day rotation of the warped current disk or a temporal up-down movement of the latter. The period in which ∂¦Dst¦/∂t< 0 during major storms coincides approximately with the period when the solar windmagnetosphere energy coupling function ɛ becomes ≳1019 erg s−1. These conclusions do not depend on the phase of the sunspot cycle.